A display such as a retinal scanning display (RSD) system typically generates images for viewing, and such images are typically graphical or video. A graphical image, i.e., graphic, typically changes infrequently or not at all. For example, a flight-instrument graphic of cockpit instruments may overlay a pilot's view. Typically, there is little change in this graphic other than the movement of the instrument pointers or numbers. Conversely, video images are a series of frames that typically change frequently to show movement of an object or the panning of a scene. For example, a television displays video images.
One aspect of a typical display system is the system's exit pupil, which defines the “window” through which the viewer can perceive an image when the pupil of the viewer's eye is aligned with the exit pupil. In a simplistic analogy, the exit pupil is much like a keyhole in a door. If the viewer's eye pupil is aligned with the keyhole, then the light which defines the image passes through the keyhole and enters the eye through the eye pupil such that the viewer perceives the image. However, if the viewer's eye pupil moves relative to the keyhole such that the light from the keyhole does not enter the eye pupil, then the viewer will not perceive the image. In some applications the display system generates two images, each via a respective exit pupil for each eye. This can allow the viewer to see a composite image stereoscopically.
As the viewer shifts his gaze within his field of view, the physical rotation of the viewer's eye may cause the viewer's eye pupil to move relative to the exit pupil. If the viewer's eye pupil moves sufficiently far, it can move out of alignment with the exit pupil. More specifically, the viewer will perceive an image as long a portion of the eye pupil is aligned with a portion of the exit pupil. That is, the viewer will still perceive the image as long as light from the exit pupil enters the eye pupil (although the viewer's perception of the image may vary depending on the degree of alignment between the eye pupil and the exit pupil). Assuming that the diameter of a human viewer's eye pupil typically ranges from about 2 millimeters (mm) in bright light to about 7 mm in dim light and that the width and height of the exit pupil are always smaller (e.g., 1 mm) than the diameter of the viewer's eye pupil, the viewer can move his eye over a range approximately equal to the sum of the diameter of his eye pupil and the width/height of the exit pupil (e.g., 3-8 mm) without losing sight of the image. But if the viewer shifts his gaze such that his eye pupil moves beyond this range—which he often does—then he typically loses sight of the image. While this example assumes, for simplicity of explanation, that the exit pupil is smaller than the eye pupil, this is not always the case. However, the basic concepts can still apply even where the eye pupil is larger than the exit pupil.
To prevent the viewer from losing sight of an image as he shifts his gaze, an RSD system may include a tracking display system to track the movement of the viewer's eyes and to move the exit pupils to keep them aligned with the respective eye pupils. An example of a tracking RSD is disclosed in commonly assigned International Publication WO 01/33282, filed Oct. 29, 1999, which is incorporated herein by reference.
A common problem with a tracking display system is that it may allow a viewer to perceive visual artifacts when he shifts his gaze. A visual artifact is an undesired phenomenon that a viewer perceives in an image. For example, flicker, which is a rapid fluctuation in brightness, is a visual artifact that a viewer may perceive in an image, particularly a raster-scanned image. Because a viewer's eyes can typically move faster than the display system can track the movement—a viewer's eyes can typically rotate at angular velocities up to 500°/second—there is often a slight delay between the time when the eye pupils attain their new positions and the time when the respective exit pupils become realigned with the eye pupils. During this period of misalignment, the viewer may perceive that a composite image is flickering, particularly if the display system is a raster-scanning type of display. Specifically, during the period of misalignment, light from the composite image does not enter the eye pupils, and thus the image can “disappear” until the display system realigns each exit pupil with its corresponding eye pupil. Thus, the viewer may perceive this momentary “disappearance” and the subsequent “reappearance” of the image as an artifact such as flicker.
Moreover, the viewer's perception of flicker may be exacerbated if the display produces the perceived image with raster-scanned, modulated beams of light. Because the peripheral rods and cones (responsible for peripheral vision) of the human eye have relatively fast response times, they, unlike the straight-ahead rods and cones (responsible for straight-ahead vision), may detect the flicker inherent in a scanned image if the scanning frequency is too low. Consequently, as the eye pupil and the exit pupil come into alignment, light from the exit pupil may initially strike the peripheral rods and cones. Consequently, the increased flicker sensitivity of the peripheral rods and cones may increase the viewer's perception of flicker.
Unfortunately, visual artifacts such as flicker may annoy or distract the viewer. For example, if the display system generates a flight-instrument overlay graphic, such visual artifacts may distract or irritate the pilot or slow the pilot's response time.
An expanded-exit-pupil display system can eliminate the need to track eye movements by generating one or more arrays of multiple identical exit pupils—typically an equal number of arrays for each eye—such that at least one exit pupil is always aligned with each pupil as the viewer shifts his gaze. Each array, often called an expanded exit pupil, is the region within which the individual exit pupils are located. The effective size of the expanded exit pupil is defined by the region of the eye's field of view (FOV) over which the exit pupils are distributed. This is often called the “eye box.”
Unlike the single exit pupil of the tracking RSD system discussed above, which has a single exit pupil for each eye, the cross-sectional dimensions of the expanded exit pupil are significantly larger than the diameter of the corresponding eye pupil. And ideally, the gaps between the exit pupils with the expanded exit pupil will be less than the diameter of the viewer's eye pupil. Consequently, as long as the viewer's gaze remains within the ideal expanded exit pupil, he will see the composite image because at least one of the exit pupils within each expanded exit pupil will always be aligned with the respective pupils of the viewer's eyes. Examples of a RSD system that generates an expanded exit pupil are disclosed in U.S. Pat. Nos. 5,701,132 and 6,157,352, which are incorporated by reference.
Unfortunately, generating and maintaining an ideal and sufficiently large and uniform expanded exit pupil may be difficult in many applications. Additionally, a large exit pupil may require more optical energy for a given perceived brightness. Consequently, it may be desirable to combine tracking with an expanded-exit-pupil display. An example of such a combination display system is disclosed in commonly assigned International Publication WO 01/33282, filed Oct. 29, 1999, which is incorporated by reference. Such a system may suffer from the same problems as the tracker and expanded-exit-pupil display systems discussed above.
Consequently, with many expanded exit pupils, the viewer may perceive visual artifacts even when he shifts his gaze such that it remains aligned with the expanded exit pupil. For example, if the gaps between the individual exit pupils within the expanded exit pupil are greater than the diameter of the viewer's eye pupil, then the viewer may perceive flicker as his pupil moves from a starting exit pupil to a destination exit pupil. This flicker is caused by the viewer's eye pupil losing alignment with the starting exit pupil before becoming aligned with the destination exit pupil (i.e., like moving from “keyhole” to “keyhole”). The flicker may be especially noticeable when the intensity or other characteristics of the respective images viewed through the starting and destination exit pupils are different. Furthermore, if the system is a tracking system, then the viewer may perceive additional flicker as the expanded exit pupil follows the movement of the viewer's eyes.